Do you use a chest strap? I discovered that as salt from your sweat builds up on it, it stops working. Try putting it through the washing machine, otherwise it might be terminal
Thanks for the tip, I'll give it a go.
I take mine in the shower every couple of days and wash the crap off. Seems to keep it working nicely.
Gave it a good rinse under the tap which worked for a few days. Think it needs a deeper soak after so long!